1. Field of the Invention
This invention pertains generally to position sensing, and specifically to rotary or angular position sensors which are both durable and precise for application to rugged and demanding environments.
2. Description of the Related Art
There are a variety of known techniques for angular position sensing. Optical, electrical, electrostatic and magnetic fields are all used with apparatus to measure position.
There are many known apparatus for using these energies for sensing. A few of the known apparatus are resistive contacting sensors, inductively coupled ratio detectors, variable reluctance devices, capacitively coupled ratio detectors, optical detectors using the Faraday effect, photo-activated ratio detectors, radio wave directional comparators, and electrostatic ratio detectors. There are many other known detectors, too numerous to mention herein.
These detection methods tend to each offer much value for one or more applications, but none meet all application requirements for all position sensing applications. The limitations may be due to cost, sensitivity to particular energies and fields, resistance to contamination and environment, stability, ruggedness, linearity, precision, or other similar factors.
Transportation applications generally, and specifically automotive applications, are very demanding. Temperatures may rise to 150 degrees Centigrade or more, with road contaminants such as salt and dirt splashing upon the engine compartment. This may occur while the engine is still extremely hot from operation. At the other extreme, an engine is expected to perform in the most northern climates without fault, and without special preheating.
Present throttle position sensors are manufactured using a resistive sensor combined with a sliding contactor structure. The sliding contact serves to "tap" the resistor element and provide a voltage proportional to position. The resistive sensor has proven to offer the greatest performance for cost in throttle position sensing applications, unmatched by any other technology to date. However, the resistive throttle position sensors are not without limitation.
An automotive position sensor must endure many millions or even billions of small motions referred to in the industry as dithers. These dithers are the result of mechanical motion and vibration carried into the position sensor. Additionally, during the life of a throttle position sensor, there may be a million or more full stroke cycles of motion. In resistive sensors, these motions can affect signal quality.
In spite of this shortcoming, throttle position sensors are resistive sensors. Over the years, efforts at improving the contactor-element interface have vastly improved the performance of these devices. Similar improvements in packaging and production have maintained cost advantage. A replacement component must be able to meet throttle position sensor performance requirements while offering similar price advantage.
The combination of temperature extremes and contamination to which an automotive sensor is exposed causes the industry to explore very rugged and durable components. One particular group of sensors, those which utilize magnetic energy, are rapidly being accepted into these demanding applications. This is because of the inherent insensitivity of the magnetic system to contamination, together with durability characteristic of the components.
Applying magnetic sensing to tone wheels for applications such as anti-lock braking and ignition timing has been a relatively easy task. The impulse provided by the tone wheel is readily detected through all conditions, with very simple electronic circuitry.
Magnetic position sensors, particularly those using Hall effect IC detectors, are also being pursued. Many in the industry believe these sensors will ultimately offer advantages over the present resistive technology. However, prior to the present invention, none of these sensors were able to offer the necessary combination of low cost, reliability, and precision output.
Magnetic circuits offer admirable performance upon exposure to the usual moisture and dirt contaminants. However, linearity and tight tolerances are another issue. Sensors are subjected to both radial and axial forces that change the alignment of the rotor portion of the sensor with respect to the stationary portion (stator). Somewhere in the system is at least one bearing, and this bearing will have a finite amount of play, or motion. That play results in the rotor moving relative to the stator.
Unfortunately, magnetic circuits of the prior art tend to be very sensitive to mechanical motion between the rotor and stator. As noted, this motion may be in an axial direction parallel to the axis of rotation, or may be in a radial direction perpendicular to the axis, or a combination thereof.
Typical magnetic circuits use one or a combination of magnets to generate a field across an air gap. The magnetic field sensor, be this a Hall effect device or a magnetoresistive material or some other magnetic field sensor, is then inserted into the gap. The sensor is aligned centrally within the cross-section of the gap. Magnetic field lines are not constrained anywhere within the gap, but tend to be most dense and of consistent strength centrally within the gap. Various means may be provided to vary the strength of the field monitored by the sensor, ranging from shunting the magnetic field around the gap to changing the dimensions of the gap.
Regardless of the arrangement and method for changing the field about the sensor, the magnetic circuit faces several obstacles which have heretofore not been overcome. Movement of the sensor relative to the gap, which is the result of axial and radial play between the rotor and stator, will lead to a variation in field strength measured by the sensor. This effect is particularly pronounced in Hall effect, magneto-resistive and other similar sensors, where the sensor is sensitive about a single axis and insensitive to perpendicular magnetic fields.
The familiar bulging of field lines jumping a gap illustrates this, where a Hall effect sensor not accurately positioned in the gap will measure the vector fraction of the field strength directly parallel to the gap. In the center of the gap, this will be equal to the full field strength. The vector fraction perpendicular thereto will be ignored by the sensor, even though the sum of the vectors is the actual field strength at that point. As the sensor is moved from the center of the gap, the field begins to diverge, or bulge, resulting in a greater fraction of the field vector being perpendicular to the gap. Since this will not be detected by the sensor, the sensor will provide a reading of insufficient magnitude.
In addition to the limitations with regard to position and field strength, another set of issues must be addressed. A position sensor of value in the transportation industry must be precise in spite of fluctuating temperatures. In order to gain useful output, a magnet must initially be completely saturated. Failure to do so will result in unpredictable performance. However, operating at complete saturation leads to another problem referred to in the trade as irreversible loss. Temperature cycling, particularly to elevated temperatures, permanently decreases the magnetic output.
A magnet also undergoes aging processes not unlike those of other materials, including oxidation and other forms of corrosion. This is commonly referred to as structural loss. Structural and irreversible loss must be understood and dealt with in order to provide a reliable device with precision output.
Another significant challenge in the design of magnetic circuits is the sensitivity of the circuit to surrounding ferromagnetic objects. For transportation applications a large amount of iron or steel may be placed in very close proximity to the sensor. The sensor must not respond to this external influence.
The prior art is illustrated, for example, by Tomczak et al in U.S. Pat. No. 4,570,118. Therein, a number of different embodiments are illustrated for forming the magnetic circuit of a Hall effect throttle position sensor. The Tomczak et al disclosure teaches the use of a sintered samarium cobalt magnet material which is either flat, arcuate, and slightly off-axis, or in second and third embodiments, rectangular with shaped pole pieces. The last embodiment is most similar to the present invention, where there are two shaped magnets of opposite polarity across an air gap of varying length.
No discussion is provided by Tomczak et al for how each magnet is magnetically coupled to the other, though from the disclosure it appears to be through the use of an air gap formed by a plastic molded carrier. Furthermore, no discussion is provided as to how this magnetic material is shaped and how the irreversible and structural losses will be managed. Sintered samarium cobalt is difficult to shape with any degree of precision, and the material is typically ground after sintering. The grinding process is difficult, expensive and imprecise. The device may be designed to be linear and precise at a given temperature and a given level of magnetic saturation, presumably fully saturated. However, such a device would not be capable of performing in a linear and precise manner, nor be reliable, through the production processes, temperature cycling and vibration realized in the transportation environment.
Furthermore, devices made with this Tomczak et al design are highly susceptible to adjacent ferromagnetic objects. The variation in adjacent ferromagnetic material from one engine to the next will serve to distort the field and adversely affect both linearity and precision. The open magnetic circuit not only adversely affects sensitivity to foreign objects, but also sensitivity to radiated energies, commonly referred to as Electro-Magnetic Interference (EMI or EMC).
The Tomczak et al embodiments are very sensitive to bearing play. The combination of an open magnetic circuit and radially narrow permanent magnet structure provides no tolerance for motion in the bearing system. This motion will be translated into a changing magnetic field, since the area within the gap in which the field is parallel and of consistent magnetic induction is very small.
Ratajski et al in U.S. Pat. No. 3,112,464 illustrate several embodiments of a brushless Hall effect potentiometer. In the first embodiment they disclose a shaped, radially magnetized structure which varies an air gap between the magnetic structure and a casing, not unlike the last embodiment of the Tomczak et al patent mentioned above. However, there is no provision for radial or axial motion of the magnet carried upon the rotor. Furthermore, the large magnetic structure is difficult to manufacture and relatively expensive.
Wu in U.S. Pat. No. 5,159,268 illustrates a shaped magnet structure similar to Ratajski et al. The structure illustrated therein suffers from the same limitations as the Ratajski et al disclosure. Additionally, the device of the Wu disclosure offers no protection from extraneous ferromagnetic objects.
Alfors in U.S. Pat. No. 5,164,668 illustrates a sensor less sensitive to radial and axial play. The disclosed device requires a large shaped magnet for precision and linearity. The size of the magnet structure places additional demand upon the bearing system. No discussion therein addresses magnet materials, methods for compensating for irreversible and structural losses, or shielding from extraneous ferromagnetic objects. The combination of large magnet, enhanced bearing structure, and added shielding combine to make a more expensive package.